Changes in the nature of the crude oil supply as well as the decrease of the demand for refined products and fuel oils, in the favour of lighter products of the gasoline type, have led the oil refinery industry to develop various processes in view of upgrading natural heavy oils as well as residual oils such as straight-run distillation residues or vacuum distillation residues.
As concerns the catalytic cracking processes for treating such charges, it appears that main difficulties arise from the fact that these charges contain organic substances such as asphaltenes as well as condensed polycyclic aromatic hydrocarbons which are difficult to crack to lighter molecules in order to obtain substantial yields of fractions of lower boiling points, and tend to combine with each other to produce substantial coke formations, depositing on the used catalysts, thus decreasing their activity. Moreover, the presence in these heavy oils of substantial amounts of heavy metals such for example as nickel, vanadium, iron etc. . . . is considered as detrimental, since these metals tend to poison or at least to deactivate the catalysts of the zeolite type, generally used in the fluid bed catalytic cracking, hereinafter called F.C.C.
A number of essential factors may however make possible to convert heavy products to lighter fractions with a good selectivity, while maintaining the coke formation to a minimum.
Among these factors, one of the most important consists of ensuring, at the level of the contact with the hydrocarbon charge --generally preheated and with steam addition--with the hot regenerated catalyst used in the catalytic cracking unit, such a mixing that heat transfer between the charge and the catalyst takes place as quickly and as regularly as possible. It is also of high importance to renew permanently and efficiently the regenerated catalyst at the level of the charge introduction in the reaction zone, particularly by avoiding backmixing phenomena which increase the contact time and result in a substantial decrease in the formation of light cuts while increasing the weight proportion of coke formed on the catalyst particles.
Another important phenomenon, also detrimental to a good operation of a F.C.C. unit, more particularly destined to the treatment of heavy charges, consists in the difficulty of obtaining a good radial homogeneity of the catalyst, as well at the inlet as through the reaction zone.
Concerning the selectivity of the cracking reactions generally, it is known that it is better as the flow of fluids is more similar to piston flow, which is particularly difficult to obtain in F.C.C. conventional units.